When I was 10, I was credited with an IQ of 170. By the time I was 16, it was measured at 155. When I took the Mensa exam six years ago at the age of 50, I passed with a score of 136 (that’s Cattell 3B scale, as were the other IQ tests. I did get a better 99.5th percentile score on Cattell Culture Fair, with a score of 156).
That doesn’t actually mean I got more stupid with age, just that I was quick for my age when I was young. People make all these “wow” noises about 6 year old kids getting into Mensa, but are they kidding? I would have creamed the Mensa exam when I was 6, as they give a LOT of allowance for age. By passing at the age of 50, I got no allowance for age, and so that was if you like my “correct” IQ.
However, if you got 120–130 on an invigilated adult IQ test at the age of 12, you’re probably still 120–130. Bright is bright, really. You’d be unlikely to drop from bright to average.